The X-ray spectrum of the newly discovered accreting millisecond pulsar IGR J17511−3057

Abstract

We report on an XMM-Newton observation of the accreting millisecond pulsar,\nIGR J17511-3057. Pulsations at 244.8339512(1) Hz are observed with an RMS\npulsed fraction of 14.4(3)%. A precise solution for the P_orb=12487.51(2)s\nbinary system is derived. The measured mass function indicates a main sequence\ncompanion with a mass between 0.15 and 0.44 Msun. The XMM-Newton spectrum of\nthe source can be modelled by at least three components, multicoloured disc\nemission, thermal emission from the NS surface and thermal Comptonization\nemission. Spectral fit of the XMM-Newton data and of the RXTE data, taken in a\nsimultaneous temporal window, constrain the Comptonization parameters: the\nelectron temperature, kT_e=51(+6,-4) keV, is rather high, while the optical\ndepth (tau=1.34(+0.03,-0.06)) is moderate. The energy dependence of the pulsed\nfraction supports the interpretation of the cooler thermal component as coming\nfrom the accretion disc, and indicates that the Comptonizing plasma surrounds\nthe hot spots on the NS surface, which provide the seed photons. Signatures of\nreflection, such as a broadened iron K-alpha emission line and a Compton hump\nat 30 keV ca., are also detected. We derive from the smearing of the reflection\ncomponent an inner disc radius of ~> 40 km for a 1.4 Msun neutron star, and an\ninclination between 38{\\deg} and 68{\\deg}. XMM-Newton also observed two type-I\nX-ray bursts, probably ignited in a nearly pure helium environment. No\nphotospheric radius expansion is observed, thus leading to an upper limit on\nthe distance to the source of 10 kpc. A lower limit of 6.5 kpc can be also set\nif it is assumed that emission during the decaying part of the burst involves\nthe whole neutron star surface. Pulsations observed during the burst decay are\ncompatible with being phase locked, and have a similar amplitude, than\npre-burst pulsations.\n